Tuesday, May 15, 2012

Piping for Steam Distribution

Introduction

Steam is widely used in most of the process industry as process heating media and in all power generation plants. Handling of steam is becoming very important. To study and understand the property of stem, condensate and it’s effect on the material of construction is again important while designing the piping system for steam distribution.

Steam Distribution

Typical Steam Distribution System

A typical steam distribution system consists of

1) Boiler where steam is generated . 
 
2) Steam using equipment such as turbine , evaporator, reaction vessel, ejector, etc.
 
3) Interconnecting piping . 
 
4) Pressure reducing station (PRS) with desuperheater (PRDS) or without desuperheater. 
 
5) Condensate return piping system with or without flash steam recovery system.

Out of the above, items (4) and (5) are seen when steam is used for process heating or space heating applications.

For process or space heating options other steams are available viz. Water (Sometimes at high pressure) and thermic fluids (including high temperature oils).

Annexure I

Contains following in relation to foregoing:

• Why use steam. 
 
• Comparison of steam with water and high temperature oils. 
 
• Steam utilization 
 
• Schematic diagram for a typical PRS.

Line Size for Steam Pipe


The line size for steam distribution is, an in other cases, decided by the steam flow rate and permissible pressure drop.

However in case of steam piping a few other aspects need to considered such as:

• Quality of steam (Dry saturated/Wet exhaust /Superheated) 
 
• Whether main supply line (often referred to steam header) or branch line (Pipeline form steam header to user equipment)

The Following Pages Contain Discussion on Following

• Unwin equation for calculation of pressure drop for steam service.

• Nomograph for finding equivalent length of fittings Discussion on economic velocity 
.
• Discussion on consideration applicable for sizing of steam mains (headers) and branch lines.

• Recommended practices for laying of pipelines carrying saturated steam.

• Table giving steam carrying capacity of steam pipes of different sizes.

Annexure 2

Gives details regarding another method used for determining line size for steam service. This method is based on use of concept of Pressure Factors.

Notes:

Additional observations related to the contents of following pages are given as below in the form of notes:

1) Unwin eqn. using metric units

2) Equivalent length of fitting

The nomograph given is in British units. Some Approximations can be seen as below:


Equations for pressure drop based on rigorous mathematics are available for isothermal as well as adiabatic flow conditions. However certain empirical equations based theoretical consideration have proved adequate for the practical purpose.

For steam applications equation widely used is Unwin equation :


For converting the answer to metric units :


Various units of Pressure measurement

1 atm   -   1.033 Kg / cm2  =  1.013 bar= 14.7 psi
            -   1.02 Kg / cm2  =  1 bar = 14.5 psi
            -   1 Kg / cm2  =  0.981 bar = 14.23 psi

Pressure drop through fittings such as bend, reducer ,valve etc.

Fluid suffers pressure drop as it passes through fittings.

The resistance offered by fittings can be correlated to the length of the straight pipe offering the same resistance in terms of pressure drop.

Such length of straight pipe is designated as an equivalent length of the fitting.

For any segment of the pipeline the total equivalent length (le in equation 1) is the total of length of the straight pipe plus the equivalent length of all fittings. A Nomograph for equivalent length of fittings is given.

Resistance of Valves and Fittings to Flow of Fluids
Resistance of Valves and Fittings to Flow of Fluids

Economic Velocity for Deciding Line Size

For given mass/volume flow rate of steam, increasing the pipe diametre (ID) would reduce the velocity and would mean less pressure drop. However a higher pipe size would mean higher initial cost of piping. The effect of change in the pipe size could be seen as below :

Economic Velocity

A compromise has to be reached between these opposing factors of initial cost, pressure drop and heat loss. Velocity of steam which offers optimum solution is referred to as economic velocity.

For steam service the recommended velocities are :
Exhaust wet steam 15 - 25 m /s
Dry saturated steam 25 - 35 m/s
Superheated steam 35 - 45 m/s

At these velocities the pressure drop would be about 1 psi /100 ft or about 0.2 bar / 100 m.



For purpose of deciding pipe sizes for steam service, the steam pipe –lines could be broadly grouped into two categories viz. steam mains and branch lines.

Steam mains are large size line spanning considerable distance. They have to deliver steam of the required quantity to the various steam using devices. The pressure drop is therefore an important consideration . The design procedure therefore involves selecting steam velocity closest to if not within economic range which gives pressure drop within permissible limit.

Branch lines are much shorter in length. The pressure drop therefore is not of substantial magnitude. The branch lines are therefore sized on the basis of velocity of 25 - 35 m/s .

Recommended take off point with branch drainage
Drain and Vent
Relaying to higher level
Ineffective and proper drain point
Figure Steam Line Reducer

Summing Up

Steam distribution and steam pipe sizing is different compared to normal gases. So Piping design and engineering for a steam line is a very important responsibility of the piping engineer. To understand the property of steam and it’s effect on the material of construction from the safety and cost point of view is important while sizing of lines of steam, piping element and the mounting on it affecting sizing, since they create pressure drop in the lines.

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7 comments :

  1. Rajeeva MahaarachchiMay 15, 2012 at 2:17 AM

    good thing, good lessons, Thank you for article.

    ReplyDelete
  2. Thanks Rajeeva for your comment!!

    ReplyDelete
  3. yes once again Ankit, thanks for the article. Very informative and useful. You are appreciated!

    ReplyDelete
  4. very good ,thanks 

    ReplyDelete
  5. Thanks ankit
    gud one

    ReplyDelete

 

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